The dynamic resistance of the diffusion barrier of a linear lambda probe—which may be represented as a temperature-dependent, complex reactance with several RC sections—which is located in the exhaust gas conduit of an internal combustion engine for the purpose of determining the fuel/air mix supplied to the internal combustion engine, exhibits a temperature dependency that gives rise to errors in the transfer ratio, which is to say in the measurement result. This is countered by measuring the probe temperature and regulating it to a constant value (750° C., for example) by means of a heating element mounted in the lambda probe. A separate thermo element for measuring the temperature is dispensed with here for reasons of cost; the highly temperature-dependent internal resistance Rpvs of the lambda probe is measured instead.
A known method for determining the internal resistance of the Rpvs of a linear oxygen probe (lambda probe) is to cause an alternating current with, for example, 500 μApp (peak-peak) and a frequency of 3 kHz to impinge on the probe terminal Vs+. An alternating current signal drops on the internal resistor Rpvs. When Rpvs=100Q: 500 μApp*100Ω=50 mVpp. This alternating current signal is amplified and rectified and can then be supplied to an analog/digital converter of a microprocessor in order to regulate the temperature of the oxygen probe.
The probe resistance Rpvs has a high impedance (around 1MΩ at 200° C.) during the heating phase and the amplitude of the alternating current signal dropping on it is correspondingly large (max. 5Vpp).
To allow the internal resistance Rpvs to be detected early, the amplifier (Rpvs_Amp) must have a low amplification. A typical measuring area would be 0 . . . 24*R0 (area 2: cold probe), where R0 corresponds to the nominal (set) probe resistance (100Ω at 750° C., for example). A wider measuring area spread is required in standard operation, 0 . . . 6*R0, for example (area 1: warm probe).
In known embodiments, the measuring areas are altered by changing over the amplification in the amplifier (Rpvs_Amp), for example *4 (heating phase, area 2) and *16 (standard operation, area 1). This converts the value for the probe internal resistance Rpvs (after amplification and rectification) into an output voltage in the area 0 . . . 4.8V. If an offset voltage of 0.1V is then added to this direct-current voltage, the result is an output voltage area of 0.1V . . . 4.9V. This voltage area can be processed in the rectifier (operating voltage 5V) and exploits the area of the analog/digital converter.
However, the large amplitude of the alternating current signal during the heating phase (max. 5Vpp) is a serious disadvantage of this solution. With some types of probe it can damage the ceramic (so-called blackening) so is not acceptable. A typical maximum value is approximately 2Vpp. The alternating current signal can accordingly only be applied when the probe is sufficiently warm—has low impedance.
In order nonetheless to be able to monitor the heating phase, recourse is taken to observing the pump current Ip (if the probe has sufficiently low impedance, a pump current Ip can also flow and Ip regulating stabilizes). However, this method is imprecise and requires considerable software expenditure in the microcontroller.
Another problem results from the fact that the oscillator has to be stopped when the circuit is switched into operation. Its output is applied to 0V or 5V. The probe terminal Vs+ which at this time has a very high impedance is connected to the oscillator output via resistor Rv and capacitor Cv. As capacitor Cv has been discharged, the potential on probe terminal Vs+ follows the potential of the oscillator output and is also applied to 0V or 5V.
However, this value is outside the nominal operating range. A diagnostic circuit, not represented here, detects this as a short-to-ground or short-to-battery voltage and would report it as a (non-existent) fault (phantom fault) requiring the aid of complex software measures to suppress.